1. Field of the Invention
The present invention relates to circuits including a dc/ac converter for supplying an ac consumer with an alternating voltage from a dc voltage source, and in particular to such a circuit which supplies the alternating voltage, in particular a sinusoidal voltage, independently of the nature of the ac consumer load.
2. Description of the Prior Art
Generation of alternating voltages such as sinusoidal voltages from a direct voltage source such as a battery by the employment of dc/ac converters is generally known. The purpose of such circuits is basically to insure continued supply of an alternating voltage to those consumers which require exclusively alternating voltage, such as for operating motors, or to those consumers whose standard design for ac operation cannot be modified. In the event of a mains breakdown or if no mains terminal is available, the needed alternating voltage can be supplied from a battery or other direct voltage source by the use of such converters.
The state of the art for high grade dc/ac converters is described in the periodicals "Ericsson Revue", No. 1, 1979 beginning at page 34 and "Electronics", Aug. 2, 1979, page 119 and Dec. 6, 1979, pages 69-70. The conventional dc/ac converters described in those articles contain a so-called transverter which is designed as a blocking converter or as a push-push or push-pull through flow converter which oscillates at a frequency which is substantially higher than the frequency of the desired alternating voltage. The transverter output can be modulated, such as by pulse duration modulation, by a modulation signal which corresponds to the desired alternating voltage and which is derived from a reference oscillator. In general, controlled switching elements which assume respective conducting and non-conducting states in dependence upon current direction such as, for example, thyristers. The controlled switching elements are interconnected between the transverter and the ac consumer and by virtue of the alternating operation of the switching elements the desired alternating voltage can be assembled from consecutive half waves.
Because of the presence of the controlled switching elements which operate in dependence upon current direction, conventional circuit arrangements of the type described above can transmit energy in only one direction. If the ac consumer who is to be supplied represents a complex load, distortions occur in the alternating voltage which originate from the energy which is periodically stored in reactance components of the load resistance. As is known, the energy stored periodically in the non-ohmic components of a load resistance must be periodically dissipated, that is, the energy must be nullified or returned to the source or used in another manner. The effect of the stored energy is to cause the output voltage to rise above a theoretical value of the desired alternating voltage. Magnetic energy stored in inductive reactive impedances results in a voltage excess during the rising portion of a theoretical voltage value curve, whereas electrical energy stored in capacitive load components results in a voltage excess during the falling portion of the theoretical voltage value. This latter deviation from the theoretical value is of particular importance because of the necessary presence of filter capacitors in the dc/ac converter.
One attempt to compensate capacitive or inductive reactive power is to employ appropriately dimensioned dual components, this technique being referred to as resonance tuning. Resonance tuning, however, is possible only for the fundamental wave of the alternating voltage.
In general, however, the above-described problem of compensation for complex load impedances is virtually disregarded in the design and operation of conventional dc/ac converters. Such converters are designed and operated under the assumption that a completely ohmic load is present, and distortion resulting from deviations from this ideal assumption are accepted. Apart from the fact that only the fundamental wave can be acted upon by resonance tuning, load compensation by resonance tuning has the further disadvantage that such tuning relates only to a specific given load value, and thus devices employing resonance tuning cannot be universally employed unless the compensating elements are greatly over-dimensioned. In many applications, the space and weight requirements of such over-dimensioned components constitute a significant disadvantage.